Comparative study of methods to estimate hydraulic parametersin the hydraulically undisturbed Opalinus Clay (Switzerland)

Catherine Yu1,2 • Jean-Michel Matray1 • Julio Goncalves2 • David Jaeggi3 •

Werner Grasle4 • Klaus Wieczorek5 • Tobias Vogt6 • Erik Sykes7

Received: 11 March 2016 / Accepted: 17 December 2016 / Published online: 25 February 2017

� The Author(s) 2017. This article is published with open access at Springerlink.com

Abstract The deep borehole (DB) experiment gave the

opportunity to acquire hydraulic parameters in a hydraulically

undisturbed zone of the Opalinus Clay at the Mont Terri rock

laboratory (Switzerland). Three methods were used to esti-

mate hydraulic conductivity and specific storage values of the

OpalinusClay formation and its bounding formations through

the 248 m deep borehole BDB-1: application of a Poiseuille-

type law involving petrophysical measurements, spectral

analysis of pressure time series and in situ hydraulic tests. The

hydraulic conductivity range in theOpalinusClaygivenby the

first method is 2 9 10-14–6 9 10-13 m s-1 for a cementa-

tion factor ranging between 2 and 3. These results show low

vertical variability whereas in situ hydraulic tests suggest

higher values up to 7 9 10-12 m s-1. Core analysis provides

economical estimates of the homogeneous matrix hydraulic

properties but donot account for heterogeneities at larger scale

such aspotential tectonic conductive features. Specific storage

values obtained by spectral analysis are consistent and in the

order of 10-6 m-1, while formulations using phase shift and

gain between pore pressure signals were found to be inap-

propriate to evaluate hydraulic conductivity in the Opalinus

Clay.Thevalues obtainedare globally ingoodagreementwith

the ones obtained previously at the rock laboratory.

Keywords Argillaceous formation � Hydraulic well tests �Poiseuille-type law � Harmonic tidal analysis � Hydraulicconductivity � Specific storage � Nuclear waste disposal

1 Introduction

Based on favourable confining properties, such as low

permeability, strong retention and self-sealing capacities,

clay formations are the preferred host rock option for a

deep geological repository of long-lived, intermediate and

high level radioactive waste in several countries including

France, Belgium and Switzerland. In the latter country, the

Opalinus Clay (OPA) has been selected as a potential host

rock for a disposal facility (Nagra 2002) and has been

studied at the Mont Terri rock laboratory since 1996. The

laboratory is located at a depth of ca. 280 m, in the security

gallery of the A16 Transjurane motorway, which crosses

the Jura Mountains in north-western Switzerland.

The accurate hydraulic characterisation of low perme-

ability formations is of high importance to ensure the safety

of a geological repository. Hydraulic properties can be

estimated by various laboratory and field experiments (Van

Editorial handling: P. Bossart and A. G. Milnes.

This is paper #4 in the Mont Terri Special Issue of the Swiss Journal

of Geosciences (see Bossart et al. 2017, Table 3 and Fig. 7).

& Catherine Yu

catherine.jiyu@irsn.fr

1 Institut de Radioprotection et de Surete Nucleaire, 31 Allee

du General Leclerc, 92260 Fontenay-aux-Roses, France

2 Aix Marseille Universite UMR 6635 CEREGE Technopole

Environnement Arbois-Mediterranee, BP80,

13545 Aix-en-Provence Cedex 4, France

3 Federal Office of Topography Swisstopo, Seftigenstrasse 264,

3084 Wabern, Switzerland

4 Federal Institute for Geosciences and Natural Resources

(BGR), Stilleweg 2, 30655 Hannover, Germany

5 Global Research for Safety (GRS), Schwertnergasse 1,

50667 Cologne, Germany

6 National Cooperative for the Disposal of Radioactive Waste

(Nagra), Hardstrasse 73, 5430 Wettingen, Switzerland

7 Nuclear Waste Management Organization, 22 St. Clair Ave.

E., Toronto, ON, Canada

Swiss J Geosci (2017) 110:85–104

DOI 10.1007/s00015-016-0257-9

der Kamp 2001; Yu et al. 2013), including empirical

methods based on the rock matrix properties (Chapuis and

Aubertin 2003), falling head or constant head permeameter

tests in laboratory (Boulin et al. 2012), and in situ field tests

that rely on measurement of pore pressure or water level

changes due to tidal natural loading (Bredehoeft 1967;

Merritt 2004; Jiang et al. 2013) or artificial application of

an hydraulic pressure different from the static formation

pressure (Neuzil 1982; Butler 1998; Mejıas et al. 2009). As

these methods are carried out from sub-millimetre to hec-

tometre investigation scales, scale dependency can affect

the results (Keller et al. 1989; Neuzil 1994).

This paper compares three different techniques to esti-

mate hydraulic properties of the Opalinus Clay: application

of a Poiseuille-type law involving petrophysical measure-

ments, in situ packer tests and spectral analysis of pore

pressure time series.

2 Geological setting

The Opalinus Clay at the Mont Terri site is an overconsoli-

dated claystone of Aalenian-Toarcian age, overlain by 800 m

of Middle to Late Jurassic limestones, marls and shales, and

underlain by 400 m of Early Jurassic to Triassic marls and

limestones, dolomites and anhydrites (Fig. 1). The thickness

of the Opalinus Clay in the Mont Terri anticline varies

between 130 m in the BDB-1 borehole and 160 m at rock

laboratory level, depending on the tectonic contribution. This

corresponds to a sedimentary thickness of about 120 m, when

corrected for tectonic overthrusting. The Opalinus Clay

reached a burial depth of 1350 m about 120 Ma ago during

early Cretaceous, which resulted in a maximum temperature

of 80–90 �C (Mazurek et al. 2006). A period of marine

regression occurred between 100 and 40 Ma, leading to a

subaerial exposure of the top of the Malm limestone. Starting

about 40 Ma, the rifting of the Rhine Graben affected

Northern Switzerland, resulting in considerable subsidence of

the area in themid-Tertiary, which brought the Opalinus Clay

sequence back to about 500 m depth. Two sea invasions into

theMont Terri area took place during Priabonian (37–34 Ma)

and during theRupelian (34–28 Ma) (Clauer et al. 2017). Late

Alpine folding during the late Miocene to Pliocene (about

12–3 Ma) formed the Folded Jura. Erosion exposed the core

of theMont Terri anticline towards 2.5 Ma, and allowed fresh

water infiltration to theMiddle Jurassic limestones. Similarly,

infiltration to the Early Jurassic limestoneswould have started

in the Quaternary, around 350 thousand years ago (Pearson

et al. 2003).

Three main facies were identified within the Opalinus

Clay (Blaesi et al. 1991): a shaly facies in the lower part of

the sequence, a thin carbonate-rich sandy facies in the

Fig. 1 Geological cross-section of the Mont Terri anticline. Location

of the rock laboratory is indicated by a white line. The BDB-1 deep

borehole, represented by a thick black line, crosses the lower part of

the Dogger aquifer, the entire Opalinus Clay formation and the upper

part of the Liassic marls (adapted from Nussbaum et al. 2017)

86 C. Yu et al.

middle part of the formation, and a sandy facies inter-

stratified with shaly facies in the upper sequence. The shaly

facies mineral composition includes 27–78% of clay min-

erals (illite, chlorite, kaolinite and illite–smectite mixed

layers), 4–29% of carbonates, 10–32% of quartz, and

accessory feldspars, pyrite and organic matter (Bossart and

Thury 2008).

Several minor tectonic faults and a larger fault zone

called ‘‘Main Fault’’ can be observed in the Opalinus Clay

(Nussbaum et al. 2011). Nagra’s investigations in deep

boreholes at Riniken, Weiach, Schafisheim and Benken

revealed that the tectonically disturbed zones are

hydraulically similar to the undeformed matrix (Johns et al.

1995; Gautschi 2001). Based on permeameter tests and

in situ packer tests, hydraulic conductivity values in tec-

tonically disturbed zones are in the range of 2 9 10-14 to

2 9 10-12 m s-1, and specific storage ranges from

2 9 10-7 to 1.7 9 10-4 m-1 (Marschall et al. 2005).

3 BDB-1 deep borehole

The deep borehole experiment (DB) aims at evaluating the

hydrogeological properties and processes of undisturbed

Opalinus Clay at the Mont Terri rock laboratory. For the

Fig. 2 a Stratigraphic sequence along the BDB-1 borehole; b BDB-1 borehole layout; c Layout of double packer elements (adapted from

Hostettler et al. 2017)

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 87

first time in this laboratory, a 247.5 m long 45� downwardinclined borehole has been drilled through the Opalinus

Clay and the bounding formations. The stratigraphic

sequence crossed by the borehole is presented in Fig. 2a

and is described in detail in Hostettler et al. (2017). The

borehole was entirely cored for stratigraphic, petrophysi-

cal, mineralogical and geochemical studies. The Opalinus

Clay section was drilled with air as drilling fluid. Drilling

was immediately followed by the installation of a multi-

packer system (Fierz and Rosli 2014) consisting in five

double packer measuring intervals and an interval port

within the Opalinus Clay, a single packer in the Staffelegg

Formation at the bottom of the borehole, and a further

double packer interval isolating the lowermost zone of the

Passwang Formation (Fig. 2b, c). Intervals were equipped

with sensors that enable long term monitoring of pressure

and temperature (Table 1). Pressure sensors are located at

the surface and connected by stainless steel lines to the

interval fluids, whereas temperature sensors are located

downhole inside the intervals.

4 Techniques for hydraulic parameters evaluation

4.1 Petrophysical model

Assuming a plane-parallel geometry, the intrinsic perme-

ability can be computed across an argillaceous formation

using a semi-empirical Poiseuille-type law (Kostek et al.

1992; Pape et al. 1999; Tremosa 2010):

k ¼ b2

3F; ð1Þ

where k is the intrinsic permeability [m2], b is the half-pore

size [m] and F is the formation factor [-], which accounts

for the tortuosity of the porous media and can be deter-

mined using the Archie’s law (Archie 1942):

F ¼ x�m; ð2Þ

where x is the porosity [-] and m is the cementation

factor. The formation factor can also be related to diffusion

parameters (Boving and Grathwohl 2001; Van Loon and

Mibus 2015) or electrical properties (Archie 1942), fol-

lowing Eqs. (3) and (4):

F ¼ Dw

De

; ð3Þ

where Dw is the diffusion coefficient in pure water [m2 s-1]

and De is the effective diffusion coefficient [m2 s-1].

F ¼ R0

Rw

; ð4Þ

where R0 is the rock resistivity [ohm m] saturated with a

brine of resistivity Rw [ohm m].

The half-pore size can be computed from petrophysical

parameters according to the following relation based on a

mass balance equation (Neuzil 2000; Altinier 2006):

b ¼ xð1� xÞqsAs

; ð5Þ

where b is the half-pore size [m], x is the porosity [-], qsis the grain density [g m-3] and As is the specific surface

area [m2 g-1].

Intrinsic permeability and hydraulic conductivity are

linked according to:

K ¼kqf g

lf; ð6Þ

where K is the hydraulic conductivity [m s-1], qf is the

fluid density [kg m-3], g is the gravity acceleration [m s-2]

and lf is the fluid dynamic viscosity [Pa s].

Fluid dynamic viscosity was estimated according to

Mercer et al. (1975):

lf ¼ ð5:38þ 3:8A� 0:26A2Þ � 10�3A; ð7Þ

with

A ¼ T � 150

100; ð8Þ

where lf is the fluid dynamic viscosity [Pa s] and T is the

temperature [�C].The Unesco equation of state (1981) was used to

determine the fluid density as a function of salinity, tem-

perature and pressure.

Determination of petrophysical parameters were per-

formed in laboratory on representative element volume

samples taken from the central part of BDB-1 drillcores.

Porosity and water contents were determined by weighing

before and after oven-drying at 105 �C until mass stabilisa-

tion. Density and degree of saturation were calculated based

on Archimede’s principle after sample immersion into ker-

dane following the experimental protocol first proposed by

Monnier et al. (1973) and later adapted by Matray et al.

(2007) for argillite samples. Grain density was evaluated

using a helium pycnometer (Micromeritics� AccuPyc II

1340) on oven-dried samples and also recalculated from

results of X-Ray diffraction measurements on bulk samples.

Table 1 Specifications of the pressure and temperature sensors

installed in BDB-1 borehole

Sensor type Temperature Pore pressure

Model IST AG PT1000 Keller AG PAA-33X

Validity range -50 to 650 �C 0–50 bars (absolute)

Accuracy ± (0.15 ? 0.002 |T|) �C 0.05% FS

88 C. Yu et al.

4.2 In-situ hydraulic testing experiments

Hydraulic in situ testing in boreholes, also referred as well

testing, is the most common method used in groundwater

and oil industries to acquire the hydraulic properties of

geological formations. Pulse withdrawal tests and constant

rate withdrawal tests were conducted in BDB-1 borehole,

from March 11th to November 16th 2015. During a with-

drawal pulse test, pressure is lowered abruptly by opening

and closing the downhole shut-in valve (Bredehoeft and

Papadopoulos 1980; Neuzil 1982). These tests are pre-

ferred as initial phase because they give an immediate

measurement of the system compressibility and generally

require shorter time frame than pumping tests. Given its

quick hydraulic response, performing more pulse tests on

interval 1 (Staffelegg Formation, Fig. 2b) was possible,

whereas two pulse tests were carried out on each of the

other intervals.

Constant rate withdrawal test parameters such as flow rate

and flow duration must be chosen with caution. In low per-

meability media, high flow rates can lead to desaturation of

the measuring intervals and extreme drops in pressure.

Therefore, a flowmeter able to sustain a very low pumping

rate of 0.3 g h-1 for several days (Bronkhorst� l-flow L01)

was used to test intervals 2 to 7 (Fig. 2b), for which the

hydraulic responses to pulse testing were the slowest.

Interval 1 was tested with a higher flow rate of 5 ml min-1

using a Bronkhorst� Liqui-Flow L10. Experimental setups

for both kind of tests and associated hydraulic responses are

respectively reported in Figs. 3 and 4. Flowmeter failed

during the testing of intervals 2 and 6 and approximatively

two months of pressure recovery were required before per-

forming a second test on these test chambers.

Hydraulic test data were analysed using the well-test

interpretation program nSIGHTS, which was developed by

INTERA for Sandia National Laboratories (Beauheim and

Roberts 2004). The code is based onBarker’s equation (1988),

which describes flow in an n-dimensional space, and does not

restrict to integer dimensions (Beauheim et al. 2004).

Ssoh

ot¼ K

rn�1

o

orrn�1 oh

or

� �; ð9Þ

where Ss is the specific storage coefficient [m-1], h is the

hydraulic head [m], t is the elapsed time [s], K is the

Fig. 3 Experimental set up for: a pulse withdrawal tests; b constant

rate pumping tests performed on BDB-1 borehole (�Solexperts)

Fig. 4 Records of pore pressure responses in the seven intervals of BDB-1 borehole to: a pulse tests; b constant rate withdrawal tests

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 89

hydraulic conductivity [m s-1], r is the radial distance

from borehole [m], and n is the flow dimension [-].

The flow area is defined as

AðrÞ ¼ b3�n 2pn=2

Cn=2rn�1; ð10Þ

where b is the extent of the flow zone [m], and C is the

gamma function [-].

nSIGHTS is able to take account of borehole inclination

by adjusting parameters such as formation thickness or

capacitive effect. Equations are programmed as functions

of pressure and the code uses unsensitively pressure or

hydraulic head data according to user configuration. Flow

is simulated in saturated conditions under a pressure gra-

dient between the well and the external boundary of the

model following Dupuit conditions. Density effects do not

intervene directly in the equation system, as density is

considered constant in the test interpretation.

Uncertainties associated with the fitted parameters are

evaluated by performing random perturbation analyses.

Plausibility ranges for fitted parameters were defined prior

to the parameter optimisation procedure (Table 2). During

the inverse parameter estimation, nSIGHTS provides best-

fit results within these pre-defined ranges. Optimisation

was performed using a simplex approach. Uncertainties

associated with the fitted parameters are evaluated by

performing random perturbation analyses (not detailed in

this paper).

4.3 Tidal analysis on pore pressure time series

Rotational and gravitational forces exerted by the sun and

the moon on the Earth induce latitudinal and longitudinal

strains within the solid matrix and cause deformations with

two dominant periods: diurnal and semi-diurnal. The tidal

gravitational potential can be resolved into a finite set of

tidal components described as harmonics, which are sinu-

soidal functions of given amplitude and frequency

(Doodson and Warburg 1941; Cutillo and Bredehoeft

2011). Five main components account for about 95% of the

tidal potential: the M2 and N2 semidiurnal lunar tides, the

S2 semidiurnal solar tide, the O1 diurnal lunar tide, and the

K1 diurnal lunar-solar tide.

Seasonal or climatic variations, anthropogenic activities

and tidal forces induce hydraulic pressure changes in

geological formations. The amplitude of the pressure

response depends on the poroelastic response of the aquifer

matrix. Pressure signal can therefore be analysed to

determine hydrogeological properties, such as specific

storage, effective porosity and hydraulic conductivity. The

models used in this work are based on Terzaghi’s (1936)

effective stress concept, which assumes a constant total

stress distributed between grains and fluid stress. Brede-

hoeft (1967) related tidal strain to specific storage:

Ss ¼Dej jDhj j ; ð11Þ

where Ss is the specific storage [m-1], |De| is the amplitude

of volumetric strain fluctuation fixed at 2 9 10-8 for the

M2 tide (Melchior 1978), and |Dh| is the amplitude of rel-

ative pressure head fluctuations [m].

Jacob‘s (1940) formula was used to compute the

porosity:

x ¼ EWSsB

qf g; ð12Þ

where x correspond to the porosity [-], EW is the stiffness

modulus of water, equal to 2.05 GPa, Ss is the specific stor-

age [m-1], B is the barometric efficiency [-], which reflects

the elastic response of the system, qf is the fluid density, andg is the gravity acceleration equal to 9.81 m s-2.

Hydraulic conductivity was estimated with formulations

using the M2 harmonic amplitude and phase shift (Boldt-

Leppin and Hendry 2003; Timms and Acworth 2005),

measured at two depths, z1 and z2 [m]:

KAmpt fM2

ð Þ ¼ Ss fM2ð Þ z1 � z2ð Þ2

fM2ð Þ�1

lnAz1 fM2

ð ÞAz2 fM2

ð Þ

� �� ��2

ð13Þ

KAmpt fM2

ð Þ ¼ Ss fM2ð Þ z1 � z2ð Þ2

fM2ð Þ�1

lnAz1 fM2

ð ÞAz2 fM2

ð Þ

� �� ��2

; ð14Þ

where KAmpt is the ‘‘amplitude effective hydraulic conduc-

tivity’’, Az1 and Az2 [kPa], are the M2 earth tide amplitude

associated to the sensors, Ss [m-1] is the arithmetic mean of

the effective specific storage coefficients obtained indi-

vidually for the two sensors, fM2 [s-1] is the frequency of

the M2 earth tide equal to 2.236 9 10-5 Hz, KDut [m s-1]

is the ‘‘phase effective hydraulic conductivity’’, and Du[rad] is the spectral phase shift between the sensors.

Spectral analysis of BDB-1 borehole pressure dataset

was performed using the Multi-Statistical Analysis Tool

(MuSTAT v1), jointly developed by the Institut de

Table 2 Plausibility ranges set in nSIGHTS for fitted parameters

Fitted parameter Plausibility range

K [m s-1]

Interval 1 10-13–10-8

Interval 2 to 7 10-13–10-11

Ss [m-1] 10-8–10-4

Flow dimension [-] 1–3.5

Skin thickness [cm] 0.5–30

External boundary radius [m] 0–5

K stands for hydraulic conductivity and Ss for specific storage. Skin

zone conductivity ranges were set one order of magnitude higher

compared to intact rock

90 C. Yu et al.

Radioprotection et de Surete Nucleaire and the Institut

National Polytechnique de Toulouse (Fatmi et al. 2008;

Ababou et al. 2012; Bailly et al. 2014). Consisting in a

Python code associated with toolboxes programmed in

Matlab, the package provides automatic features: (a) pre-

processing of time series, that enables the detection of time

gaps and spurious values, as well as data reconstruction by

autoregressive first order process; (b) processing of a single

time series; (c) cross-analysis of two time series.

5 Results at various scales of investigation

5.1 Sub-millimeter to centimeter scale

5.1.1 Petrophysical parameters

The petrophysical parameters necessary for the computa-

tion of intrinsic permeability are presented in Fig. 5 as a

function of distance along BDB-1 borehole.

The mean water accessible porosity is 13.0% in the

Opalinus Clay, with a lower average porosity of 12.0% in

the sandy facies compared to the shaly facies, which

exhibit a mean porosity of 13.5%. These values are lower

than the mean value of 18% suggested by previous studies

performed at the Mont Terri tunnel level. The Passwang

Formation presents slightly lower porosity values ranging

between 8.1 and 14.6% with a mean value of 12.2%. The

Hauptrogenstein is characterised by the lowest porosity

with a mean value of 3.9%.

Grain densities obtained by helium pycnometry have a

mean value of 2.74 g cm-3 in the Opalinus Clay overlying

formations and of 2.72 g cm-3 in the argillaceous layer.

The lowest grain densities are found in the bituminous

Rietheim Member of the Staffelegg Formation (see

Fig. 1a), ranging between 2.3 and 2.4 g cm-3. These low

values are probably linked to the presence of organic

matter.

The Passwang Formation, which directly overlays the

Opalinus Clay, does not reveal clear petrophysical dis-

crepancies with the clay formation except for the specific

surface area. This parameter has an average value of

13 m2 g-1 in the carbonated section of the borehole and

shows significant fluctuations linked to the marly compo-

sition of the Passwang Formation. A higher mean value of

29 m2 g-1 characterises the Opalinus Clay.

The Opalinus Clay is also characterised by a low pore

size. Analyses of nitrogen adsorption and desorption iso-

therms show that 70 to 93% of the connected porous net-

work is constituted of mesopores (pore diameter between 2

and 50 nm), with a mean size of 13 nm. Calculation of the

half-pore size from petrophysical parameters, following

Fig. 5 Petrophysical parameters acquired along BDB-1 borehole: a Water accessible porosity acquired by oven-drying at 105 �C; b Specific

surface area obtained by BJH and BET methods; c Grain density estimated by helium pycnometry on oven dried samples

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 91

Eq. (5), gives mean pore sizes in the range of 3.1 to

7.3 nm.

Ranging between 1.3 and 5.4 (Horseman et al. 1996),

the cementation factor was estimated to be close to 2 for

compacted and deeply buried sediments (Ullman and Aller

1982; Tremosa 2010). Van Loon et al. (2003) related the

effective diffusion coefficient of tritium measured in the

Opalinus Clay to its porosity using a cementation factor of

2.5. An attempt was made to compute cementation factors

from conductivity values obtained by borehole logging in

BDB-1 and water-accessible porosity determined at labo-

ratory scale. No real conductivity of formation fluid was

acquired in the Opalinus Clay, as this part of the borehole

was drilled with air. Therefore, fluid conductivity values

were estimated based on chlorinity data acquired on BDB-

1 core samples (not detailed in this paper). Low values of

cementation factors are thus obtained and range between

0.9 and 1.7.

5.1.2 Intrinsic permeability and hydraulic conductivity

The intrinsic permeability profiles (Fig. 6a) show a low

vertical variability through the Opalinus Clay, where it

ranges between 1.8 9 10-21 and 6.1 9 10-20 m2 if a

cementation factor varying between 2 and 3 is taken. For a

cementation factor of 2.5, the mean intrinsic permeability

is 7.7 9 10-21 m2 for the Opalinus Clay shaly facies and

7.9 9 10-21 m2 for its sandy facies. These values are in

good agreement with the range of 1 9 10-21 and

6 9 10-20 m2 obtained by gas injection experiments per-

formed at the Mont Terri laboratory (Marschall et al.

2005). Based on the same cementation factor, difference

can be seen in the carbonate-rich sandy facies, where

values are about three times higher than in the shaly and

the sandy facies. With a higher exponent m = 3, the

resulting intrinsic permeability has a mean value of

7.6 9 10-21 m2 and no clear distinction arises between the

different facies. The intrinsic permeability values com-

puted in the Passwang Formation and the Staffelegg For-

mation are much more heterogeneous and vary between

1.5 9 10-21 and 5.8 9 10-20 m2.

The corresponding hydraulic conductivity profiles are

presented in Fig. 6b and show similar trends compared

with the intrinsic permeability profiles. The hydraulic

conductivity obtained for the Opalinus Clay ranges

between 1.9 9 10-14 and 5.8 9 10-13 m s-1 for a

cementation factor varying between 2 and 3. For a

cementation factor of 2.5, the formation is characterised by

a mean hydraulic conductivity of 8.3 9 10-14 m s-1. No

clear discrepancy between the shaly facies and the sandy

Fig. 6 a Intrinsic permeability profile and b hydraulic conductivity

profile computed across the Opalinus Clay (OPA) and the Passwang

Formation for cementation factor (m) of 2, 2.5 and 3. Square symbols

represent values for variable m computed based on conductivity

logging measurements across the Opalinus Clay

92 C. Yu et al.

facies is revealed, with respective mean values of

7.3 9 10-14 and 6.9 9 10-14 m s-1. These values are

consistent with the range of 2 9 10-14–1 9 10-12 m s-1

reported in previous studies (Bossart and Thury 2008). The

Passwang Formation and the Staffelegg Formation present

a various range of hydraulic conductivities between

1.6 9 10-14 and 6.1 9 10-13 m s-1.

The computation of intrinsic permeability using variable

cementation factors in the Opalinus Clay gives higher

values in the range of 4.0 9 10-20 to 1.9 9 10-19 m2,

corresponding to hydraulic conductivity values in the range

of 4.1 9 10-13–1.7 9 10-12 m s-1.

5.2 Decimeter to meter scale: in situ hydraulic tests

results

Pore pressure should be fully recovered from artificial

disturbance induced by the installation procedure (e.g.,

drilling, logging, equipment installation) before starting a

hydraulic test. Steady state was considered to be reached

when the tidal components were detected on all pore

pressure time series acquired in BDB-1 borehole, which

indicate that the system is fully pressurised and saturated

(see Sect. 5.3.1).

The observed compressibility of the test zone (Ctz) was

deduced from pulse tests and computed according to:

Ctz ¼1

Vtz

dV

dP; ð14Þ

where Vtz [m3] is the shut-in volume, dV [m3] is the

withdrawn volume and dP [Pa] is the pressure variation.

Test zone compressibility in BDB-1 borehole varies

9.1 9 10-10 and 2.4 9 10-9 Pa-1 (Fig. 7), approximately

up to a factor of 5 larger than water compressibility, which

is equal to 4.8 9 10-10 Pa-1 at 10 �C (Kell 1975). The

discrepancy can be attributed to the mechanical compliance

of the equipment.

Semi-logarithmic plots presented in Fig. 8 give a qual-

itative comparison of the hydraulic behaviours character-

ising the different tested intervals. Degree of pore pressure

dissipation (U) and normalised drawdown pressure (Unorm)

are respectively defined by the following equations:

U ¼ Ut � U0

Umin � U0

ð15Þ

Unorm ¼ Ut � Umin

U0 � Umin

; ð16Þ

where Ut [kPa] is the pore pressure at time t, U0 [kPa] is the

hydrostatic pore pressure in situ and Umin [kPa] is the pore

pressure reached after pulse application or at the end of the

pumping phase.

Discrepancies in the degree of dissipation can be

observed between tests performed on a same interval

(Fig. 8a). Constant rate withdrawal tests were carried out

using the same flow rate of 0.3 g h-1 for different dura-

tions. To compare the evolution of pore pressures in the

measuring intervals during pumping phase, Pmin was taken

to correspond to the shortest pumping duration in the cal-

culation of Unorm. If specific storage is assumed homoge-

neous through the Opalinus Clay, the order from left to

right on Fig. 8b gives an indication of decreasing

permeability.

The application of a composite model, which takes into

account a damaged skin zone, was required for most of the

test numerical interpretations. Taking as an example the

first pulse test carried out on BDB-1 Interval 2, Fig. 9

shows a comparison of the residuals (measured value

minus simulated value) to that of a normal distribution,

using a homogeneous model and a composite one. The

homogeneous model appears to be unsatisfactory because

the residuals are not normally distributed, which indicates

the presence of a systematic error.

Pulse tests and constant rate pumping tests results are

respectively compiled in Table 3. Pulse testing revealed

the highest hydraulic conductivity values in the Staffelegg

Formation (Interval 1, see Fig. 2b) with best fit values

ranging from 2.1 9 10-10 to 5.9 9 10-10 m s-1. Located

in the basal shaly facies of Opalinus Clay (Interval 2), the

bottom part of the main fault zone is characterised by

conductivity values from 3.1 9 10-12 to

7.3 9 10-12 m s-1 and do not differ from the upper shaly

facies represented by Interval 4 and 5 (Fig. 10), whose

best estimates are up to 4.2 9 10-12 m s-1. The lowest

values are found in the sandy facies (Interval 6, best fit

values up to 2.7 9 10-13 m s-1), and the carbonate-rich

sandy facies (Interval 3, best fit values up to

5.1 9 10-13 m s-1). The basal part of the Passwang

Fig. 7 System compressibility computed from pulse testing in BDB-

1 borehole. Dashed line represents the water compressibility at 10 �C.The outlier in the lower part of the borehole is due to a very low

withdrawn volume

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 93

Formation, represented by Interval 7, shows similar

hydraulic conductivity values to Opalinus Clay

(5.8 9 10-13–1.4 9 10-12 m s-1).

The analyses results of the constant flowrate withdrawal

tests are quite consistent with those obtained from pulse

tests. Indeed, a similar trend can be observed with slightly

Fig. 9 Example of residual plots for the optimization of Interval 2 (Opalinus Clay shaly facies) pulse sequence fit to the Cartesian pressure

response using a an homogeneous model and b a composite model with skin

Fig. 8 Comparison of the different tests performed on BDB-1

borehole: a degree of dissipation associated to the recovery phases

of pulse withdrawal tests; b normalised pressure drawdown during

constant rate withdrawal tests and c degree of dissipation following

the end of the withdrawal phase

94 C. Yu et al.

Table 3 Parameter estimates from BDB-1 borehole pulse withdrawal tests and constant rate (CR) withdrawal tests (K [m s-1]: hydraulic

conductivity; Ss [m-1]: specific storage; n: flow dimension; ts [cm]: skin thickness)

Test Interval Ts

[cm]

K [m s-1] Ss [m-1] n

Formation Skin Formation Skin

Range Best fit Best fit Range Best fit Best fit Range Best

fit

Pulse

C1–1 I1 – 1 9 10-10–3.5 9 10-10 2.1 9 10-10 – 6 9 10-9–

6.3 9 10-81.4 9 10-8 – 1.9–2.7 2.2

C1–2 I1 – 1 9 10-11–1 9 10-7 4.2 9 10-10 – 1.4 9 10-6 – 2.0

C1–3 I1 – 1 9 10-11–1 9 10-7 5.6 9 10-10 – 6.3 9 10-8 – 2.4

C1–6 I1 – 3 9 10-11–1 9 10-8 5.9 9 10-9 – 8.3 9 10-7 – 2.0

C2–1 I2 0.5 2 9 10-12–3 9 10-11 3.1 9 10-12 7.8 9 10-12 1 9 10-7–

3 9 10-55.2 9 10-6 4.3 9 10-5 1.8–3.0 2.8

C2–2 I2 0.5 2 9 10-12–1 9 10-10 7.3 9 10-12 1.0 9 10-11 1 9 10-7–

2 9 10-53.0 9 10-6 4.7 9 10-5 1.4–2.9 2.0

C3–1 I3 0.5 1 9 10-13–3 9 10-12 5.1 9 10-13 1.6 9 10-12 5 9 10-7–

3 9 10-53.7 9 10-6 1.2 9 10-5 1.4–3.1 2.1

C3–2 I3 0.5 2 9 10-13–2 9 10-12 4.9 9 10-13 1.6 9 10-12 2 9 10-6–

3 9 10-51.1 9 10-5 1.5 9 10-5 1.4–3.4 2.5

C4–1 I4 0.5 1 9 10-12–9 9 10-12 2.3 9 10-12 5.7 9 10-12 2 9 10-6–

1 9 10-56.4 9 10-6 5.5 9 10-5 1.7–3 2.3

C4–2 I4 2.0 7 9 10-13–1 9 10-11 4.2 9 10-12 2.7 9 10-11 1 9 10-6–

2 9 10-52.2 9 10-6 9.7 9 10-6 1.5–3 2.0

C5–1 I5 0.5 4 9 10-13–4 9 10-12 1.6 9 10-12 1.4 9 10-12 3 9 10-7–

8 9 10-61.0 9 10-6 2.6 9 10-5 1.8–2.9 1.9

C5–2 I5 0.5 4 9 10-13–3 9 10-12 1.0 9 10-12 2.7 9 10-12 1 9 10-6–

3 9 10-58.5 9 10-6 4.7 9 10-5 1–3 2.5

C6–1 I6 1.5 8 9 10-14–8 9 10-13 1.9 9 10-13 1.4 9 10-11 8 9 10-7–

1 9 10-56.6 9 10-6 6.6 9 10-6 1.7–3 2.6

C6–2 I6 0.5 2 9 10-13–6 9 10-13 2.7 9 10-13 5.4 9 10-12 1 9 10-6–

2 9 10-51.7 9 10-5 2.9 9 10-5 1.7–3 2.8

C7–1 I7 0.5 3 9 10-13–4.5 9 10-12 5.8 9 10-13 3.7 9 10-13 4 9 10-7–

2 9 10-53.7 9 10-6 1.9 9 10-6 1.9–3.5 3.0

C7–2 I7 0.5 4 9 10-13–2 9 10-12 1.4 9 10-12 8.6 9 10-12 10-9–

2 9 10-51.2 9 10-6 9.4 9 10-7 2.1–2.6 2.3

CR

C1–7 I1 4.3 1 9 10-10–1 9 10-9 5.0 9 10-10 5.7 9 10-9 1 9 10-8–

1 9 10-48.2 9 10-6 6.2 9 10-5 2.0–3.0 2.1

C2–2 I2 14.2 4 9 10-13–8 9 10-11 3.9 9 10-12 3.5 9 10-11 1.2 9 10-6 9.0 9 10-5 1.4–2.7 1.95

C3–3 I3 0.7 1 9 10-13–2 9 10-12 9.9 9 10-13 1.6 9 10-12 3 9 10-8–

6 9 10-51.5 9 10-5 4.9 9 10-5 1.5–3.0 1.9

C4–3 I4 1.4 4 9 10-14–5 9 10-12 2.4 9 10-12 3.0 9 10-12 4 9 10-8–

3 9 10-41.2 9 10-5 4.0 9 10-5 1.9–3.0 2.1

C5–3 I5 1.8 7 9 10-14–9 9 10-12 8.1 9 10-13 1.5 9 10-11 1 9 10-7–

3 9 10-58.9 9 10-5 2.4 9 10-5 1.9–3.0 2.3

C6–3 I6 0.5 1 9 10-14–4 9 10-12 2.2 9 10-13 1.1 9 10-12 1 9 10-7–

2 9 10-57.7 9 10-6 9.9 9 10-5 1.5–2.9 2.5

C7–3 I7 0.6 1 9 10-13–1 9 10-12 4.4 9 10-13 8.2 9 10-13 3 9 10-8–

4 9 10-68.5 9 10-6 2.3 9 10-5 2.1–3.0 2.7

Shaded cells represent unrealistically wide range of uncertainties

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 95

higher permeability values in the shaly facies than in the

sandy facies of Opalinus Clay.

Specific storage and flow dimension estimates are highly

variable. One issue with single well hydraulic testing is that

the volume of tested rock is limited to the immediate

vicinity of the well.

5.3 Hectometer scale: tidal analysis

5.3.1 Tidal identification in BDB-1 pore pressure series

Detection of tidal components was performed on the pore

pressure time series monitored by the sensors placed in

BDB-1 borehole, with an acquisition time step set at

15 min. The four largest amplitude tidal components, O1,

K1, S2 and M2 appear on all processed signals at the exact

expected frequencies for time series between September

1st 2014 and March 10th 2015 (Fig. 11).

The form ratio is defined as the sum of the two main

diurnal component amplitudes, K1 and O1, divided by the

sum of the two main semi-diurnal component amplitudes,

M2 and S2 (Wiegel 1964). Tidal deformation through the

Opalinus Clay at Mont Terri is characterised by a form

ratio varying between 0.84 and 1.04, which indicates a

mixed, mainly semi-diurnal tide (Table 4). The maximum

value is found in the interval located in the Passwang

Formation, for which the diurnal components have slightly

higher amplitudes than the semi-diurnal ones. Except for

this interval, the M2 tide presents the highest amplitude

among the four major tides.

5.3.2 Hydraulic parameters computation

The results of specific storage coefficient computation are

given in Table 5. Specific storage values are ranging

between 1.1 9 10-6 and 1.6 9 10-6 m-1 in the Opalinus

Clay and are higher for the adjacent formations

(2.4 9 10-6 m-1 for the Lower Dogger limestone and

3.1 9 10-6 m-1 for the Staffelegg Formation). These

estimates are consistent with the range given in the liter-

ature, deduced from in situ packer tests and permeameter

Fig. 10 a Simulation of a pulse test performed on BDB-1 Interval 5 located in the upper shaly facies of Opalinus Clay and b associated Ramey

A plot with best fit parameters. Results of 200 perturbation analyses and their confidence regions (c and d)

Fig. 11 Estimated Root Mean Square spectrum of pore pressure time

series measured in BDB-1 borehole between 01/09/2014 and 10/03/

2015. The following tides are observable: principal lunar semidiurnal

tide M2 (2.236 9 10-5 Hz) and solar semidiurnal tide S2(2.315 9 10-5 Hz), lunar diurnal tides K1 (1.161 9 10-5 Hz) and

O1 (1.076 9 10-5 Hz), and the solar diurnal components S1(1.157 9 10-5 Hz) and P1 (1.154 9 10-5 Hz)

c

96 C. Yu et al.

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 97

tests for the Opalinus Clay shaly facies: between 1 9 10-7

and 1 9 10-4 m-1, with a best estimate at 2 9 10-6 m-1

(Bossart and Thury 2008).

Effective dynamic porosity values obtained using the

M2 tide (Table 6) are globally in well agreement with those

obtained from petrophysical measurements. Indeed,

coherent values between 8 and 24% are obtained by cross-

analyses of measuring intervals located in the Opalinus

Clay. Statistical analysis carried out in previous studies on

Mont Terri samples (Fatmi 2009; Bailly and Matray 2015)

revealed very low range values between 1 and 4% at the

tunnel level. These unexplained low values could be

Table 4 Amplitudes of the tidal components with associated frequencies observed on BDB-1 pore pressure time series between 01/09/2014 and

10/03/2015

Formation/associated chamber Amplitude on the RMS spectrum [bars] Form ratio

O1

(1.076 9 10-5 Hz)

K1

(1.161 9 10-5 Hz)

S2(2.315 9 10-5 Hz)

M2

(2.236 9 10-5 Hz)

Staffelegg formation/I1 5.886 9 10-4 6.326 9 10-4 3.606 9 10-4 8.353 9 10-4 1.02

OPA-shaly facies/I2 1.054 9 10-3 1.230 9 10-3 6.848 9 10-4 1.696 9 10-3 0.96

OPA-shaly facies/I2–3 1.041 9 10-3 1.192 9 10-3 7.390 9 10-4 1.823 9 10-3 0.87

OPA-carbonate-rich facies/I3 7.905 9 10-4 9.553 9 10-4 5.014 9 10-4 1.255 9 10-3 0.99

OPA-shaly facies/I4 9.560 9 10-4 1.133 9 10-3 6.838 9 10-4 1.701 9 10-3 0.88

OPA-shaly facies/I5 8.591 9 10-4 1.084 9 10-4 6.546 9 10-4 1.670 9 10-3 0.84

OPA-sandy facies/I6 8.637 9 10-4 1.205 9 10-3 5.329 9 10-4 1.278 9 10-3 1.04

Passwang formation/I7 5.200 9 10-4 7.206 9 10-4 2.825 9 10-4 6.360 9 10-4 1.35

Table 5 Specific storage coefficients (Ss) estimated from absolute pore pressure signals for BDB-1 borehole measuring intervals with corre-

sponding formations and amplitudes of pressure head fluctuations Dh

Formation Chamber Dh [bar] Dh [m] Ss [m-1]

Upper toarcian-staffelegg formation I1 8.353 9 10-4 8.52 9 10-3 2.4 9 10-6

Upper toarcian/lower aalenian-opalinus clay–shaly facies I2 1.696 9 10-3 1.73 9 10-2 1.2 9 10-6

Upper toarcian/lower aalenian-opalinus clay–shaly facies I2–3 1.823 9 10-3 1.86 9 10-2 1.1 9 10-6

Lower aalenian-opalinus clay–carbonate-rich facies I3 1.255 9 10-3 1.28 9 10-2 1.6 9 10-6

Lower aalenian opalinus clay–shaly facies I4 1.701 9 10-3 1.73 9 10-2 1.2 9 10-6

Middle aalenian-opalinus clay–shaly facies I5 1.670 9 10-3 1.70 9 10-2 1.2 9 10-6

Upper aalenian-opalinus clay–sandy facies I6 1.278 9 10-3 1.32 9 10-2 1.5 9 10-6

Upper aalenian-passwang formation I7 6.360 9 10-4 6.49 9 10-3 3.1 9 10-6

Table 6 Spectral coherence function (Coh), arithmetic mean of the specific storativity coefficient (~Ss), amplitude of the pore pressure signal 1

(Az1), and of the pore pressure signal 2 (Az2), vertical effective amplitude hydraulic conductivity ( ~KAmpt ) and vertical effective phase hydraulic

conductivity ( ~KDut ), effective dynamic porosity (x) obtained for the M2 earth tide for different couples of sensors in BDB-1 borehole

Chamber Coh ~Ss Az1 Az2 Du ~KAmpt

~KDuv

B x xwater loss

[-] [m-1] [bar] [bar] [rad] [m s-1] [m s-1] [-] [-] [-]

I1 vs. I2 0.9985 1.8 9 10-6 8.35 9 10-4 1.70 9 10-3 -0.18220 4.7 9 10-8 7.2 9 10-7 0.2520 0.09 0.18

I2 vs. I2–3 0.9992 1.1 9 10-6 1.70 9 10-3 1.82 9 10-3 0.03573 2.5 9 10-6 1.0 9 10-5 1.0350 0.24 0.15

I2–3 vs. I3 0.9986 1.3 9 10-6 1.82 9 10-3 1.26 9 10-3 0.07658 1.4 9 10-7 3.3 9 10-6 0.3949 0.11 0.13

I3 vs. I4 0.9977 1.4 9 10-6 1.26 9 10-3 1.70 9 10-3 -0.06768 2.9 9 10-7 5.8 9 10-6 4.6810 1.33 0.12

I4 vs. I5 0.9930 1.2 9 10-6 1.70 9 10-3 1.67 9 10-3 0.02158 5.7 9 10-5 4.1 9 10-6 0.4758 0.12 0.14

I5 vs. I6 0.9965 1.4 9 10-6 1.67 9 10-3 1.28 9 10-3 -0.05037 6.4 9 10-7 1.8 9 10-5 0.2889 0.08 0.13

I6 vs. I7 0.9965 2.3 9 10-6 1.28 9 10-3 6.36 9 10-3 0.50810 3.9 9 10-8 7.3 9 10-8 1.6290 0.79 0.13

Mean water-loss porosity (xwater loss) is given for comparison purposes

Shaded cells indicate spurious values

98 C. Yu et al.

related to the hydraulically disturbed state of the studied

area and desaturation phenomena.

Hydraulic conductivity values obtained in the saturated

part of the claystone by cross-analysis (Table 6) are much

higher than those obtained by other techniques. Indeed,

high conductivities ranging between 5.7 9 10-5 and

1.4 9 10-7 m s-1 are found in the Opalinus Clay. These

values are 6 to 8 orders of magnitude higher than the range

expected from literature data, suggesting that the method is

not appropriate for this formation. Discrepancies up to

three orders of magnitude between laboratory hydraulic

conductivity results and tidal analysis results were also

reported by Boldt-Leppin and Hendry (2003) who studied

the King site claystone formation (Canada). These dis-

crepancies were explained by scale factor effects and the

presence of fractured area.

Bailly and Matray (2015) performed statistical analysis

on pore pressure time series acquired in the BCD-3 bore-

hole located at the Mont Terri tunnel level. They obtained

hydraulic conductivities ranging between 1.9 9 10-10 and

7.5 9 10-11 m s-1 in the unsaturated part of the Opalinus

Clay by applying the same method on the S1 solar diurnal

tide. The M2 tide was not found in the studied pore pressure

time series due to suction conditions associated to the rock

laboratory level. The study also suggested that the struc-

tures observed in this borehole were hydraulically con-

ductive, meaning that the Opalinus Clay true permeability

should be even lower than the range given by tidal analysis.

6 Discussion

6.1 Comparability of laboratory tests and in situ

tests results

Reliable estimates of permeability and specific storage that

describe the bulk hydraulic behaviour are needed for the

evaluation of radionuclide transport in geological forma-

tions. Linking the results of laboratory tests to bulk char-

acteristics at the regional scale is not straightforward and

the meaning of measured values has to be interpreted.

Sedimentary rocks are generally associated with aniso-

tropic properties such as permeability, diffusion coefficient

and mechanical features. In the Opalinus Clay, which is an

overconsolidated clay, a moderate permeability anisotropy

ratio of 5.5 was estimated based on laboratory permeameter

tests (Munoz et al. 2003; Croise et al. 2004; Fernandez-

Garcia et al. 2007).

The petrophysical model is based on a conceptual par-

allel plane geometry which would be associated to a flow

orientation parallel to bedding planes. Since BDB-1 bore-

hole was drilled perpendicular to bedding plane, the main

solicited direction for fluid flow during hydraulic testing is

also parallel to stratification. For its part, tidal analysis is

mainly based on gravitational forces that propagate radially

from the center of the Earth and should result, given the

setting of the Mont Terri anticline, in composite values of

parallel and perpendicular to bedding permeabilities.

Although the petrophysical model may be unsuited to

carbonated formations, calculation was also performed on

the Passwang Formation and the Staffelegg Formation,

which shows similar petrophysical parameters. Another

questionable point is the use of a constant value for the

Archie’s exponent since this parameter depends on the

nature of the porous medium. Consequently, adapted val-

ues should be taken in the future according to the evolution

of rock facies along the stratigraphic sequence. Conduc-

tivities values obtained in BDB-1 with variable cementa-

tion factor (Fig. 12a) are only indicative and not quality-

assured, given the uncertainties linked to data acquisition.

Indeed, the Opalinus Clay was in the air-drilled section of

the borehole, giving constraints for in situ determination of

cementation factor.

Fitting the cementation factor by comparing petro-

physical results and estimates from hydraulic tests can be

debatable. Indeed, the volume of solicited rock is higher in

the latter case and takes greater account of formation

heterogeneities and local potentially open fractures. This

point is clearly illustrated by the discrepancies observed for

the Staffelegg Formation, in which many fractures were

evidenced by drillcore mapping. Indeed, petrophysical

measurements on centimetre-scale samples do not take into

account these hydraulically conductive structures and

underestimate the values of bulk properties.

Archie’s law is rigorously an empirical relationship that

links the electrical resistivity of saturated clay-free rocks

and their porosity. However, an analogy can be made

between the electrical potential and the concentration. It

has been shown that effective diffusion coefficient could be

predicted by this relationship in a variety of clays and

shales with a cementation factor ranging between 2 and 3

(Boving and Grathwohl 2001; Van Loon et al. 2003;

Mazurek et al. 2009). Best fit values of hydraulic con-

ductivity obtained from hydraulic testing are generally

higher that those computed with the petrophysical law

(Fig. 12a). Hydraulic conductivities higher than

10-12 m s-1 found in the Opalinus Clay shaly facies would

be associated to illogical values of cementation factor

inferior to 1.3, which was given for clean unconsolidated

sand packs by Archie (1942).

Whether it be for pulse or constant withdrawal tests, the

numerical interpretation of hydraulic tests suggests rather

wide and unrealistic ranges of uncertainties for hydraulic

conductivity and specific storage. Covering several orders

of magnitude and not tightly around the best estimates

(Table 3), these uncertainties are probably linked to the

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 99

large number of fitted parameters. Tidal analysis may be

more representative than single well hydraulic testing for

specific storage estimation (Fig. 12b), since the tidal

deformation is applied to the entire rockmass. The highest

values for specific storage obtained from pulse testing

should be taken with caution since the sensitivity to this

parameter is low for this kind of test (Cooper et al. 1967).

6.2 Consistency with previous results

Numerous in situ and laboratory investigations have been

carried out at the Mont Terri rock laboratory to charac-

terize the hydraulic properties of the Opalinus Clay. Lab-

oratory permeameter tests revealed conductivity values

ranging from 6 to 12 9 10-14 m s-1 with high associated

storage coefficient of 4.8 9 10-4 m-1 (Croise et al. 2004).

Figure 13 shows a compilation of hydraulic conductivity

results obtained from packer tests (pulse, constant head and

constant rate) performed previously at the Mont Terri site

(Lavanchy and Mettier 2012), along with data collected in

BDB-1 borehole. Tests were mainly performed in bore-

holes oblique or normal to bedding drilled in area unaf-

fected by the excavation damaged zone of the tunnel.

Previous permeability values measured at the rock labo-

ratory level range from 1.5 9 10-14 to 1.1 9 10-9 m s-1

with 55% of the values in the order of 10-13 m s-1. The

high values above 1.1•10-10 m s-1 of the shaly facies

from previous studies might be affected by the excavation

damaged zone (EDZ) and are not quality assured. The best

fit values obtained from BDB-1 hydraulic testing fall vir-

tually in the expected range with higher values in the order

of 10-12 m s-1 characterising the Opalinus Clay shaly

facies.

Specific storage coefficients obtained by tidal analysis

are rather homogeneous within the Opalinus Clay with

values in the order of 10-6 m-1, which are comparable to

the range of 2 9 10-6 to 5 9 10-6 m-1 found by Bailly

and Matray (2015).

No significant correlation between the hydraulic con-

ductivity and the different lithological facies was high-

lighted by Croise et al. (2004), Nussbaum and Bossart

(2004) and Lavanchy and Mettier (2012) due to a lack of

data from the sandy facies. Although best fit values

obtained from BDB-1 borehole indicate higher values in

the shaly facies, uncertainty ranges make it difficult to

conclude on a possible contrast. Numerical simulations

show that sandstone lenses embedded in clay rich strata do

not compromise the barrier function of the Opalinus Clay

since low hydraulic conductivity values characterise the

entire formation. Furthermore, Opalinus Clay sandy layers

are better cemented and display lower porosities (Fig. 5a).

Microscopic observations in the sandy facies revealed

precipitation of authigenic quartz, carbonates and kaolinite

(Peters et al. 2011). On the other hand, porosity values

measured in BDB-1 borehole are globally lower than those

obtained at the rock laboratory tunnel level and may reflect

Fig. 12 Comparison of results obtained by petrophysical analysis, in situ hydraulic testing and tidal spectral analysis performed on BDB-1

borehole: a hydraulic conductivity b specific storage

100 C. Yu et al.

the deconfinement and relaxation of stresses occurring at

the latter location.

The Main Fault that intersects the laboratory does not

impact the barrier function of the Opalinus Clay. Indeed,

the sealing of fault planes by calcite shear fibres and clay

minerals induce small effect of tectonic deformation on the

hydraulic properties of the Opalinus Clay (Nussbaum et al.

2011). This observation is supported by the consistency

between the hydraulic tests performed in the intact shaly

facies and those carried out in the interval crossing the fault

zone. Similarly, no contrast can be identified on the dif-

ferent profiles obtained with the petrophysical model.

7 Conclusions

The deep borehole (DB) experiment enabled the acquisi-

tion of data in a fresh borehole penetrating the entire

hydraulically undisturbed Opalinus Clay at Mont Terri.

Therefore, the presented results are unique, because other

hydraulic data at Mont Terri are or might be influenced by

tunneling and experimental activities. Three methods with

different investigation volumes were carried out and

compared.

A model that links intrinsic permeability to petrophys-

ical parameters gives intrinsic permeability values ranging

between 2 9 10-21 and 6 9 10-20 m2 for a cementation

factor varying between 2 and 3, corresponding to hydraulic

conductivities between 2 9 10-14 and 6 9 10-13 m s-1.

Tidal analysis revealed itself not to be an appropriate

method to compute hydraulic conductivity in our study,

giving values overestimated of several orders of

magnitudes. However, this approach gives reasonable

values for specific storage and effective porosity. As a third

method, in situ hydraulic testing was performed using the

multipacker system installed in BDB-1 borehole. Hydraulic

conductivity values obtained by numerical inversion from

pulse tests are consistent with those deduced from constant

rate withdrawal tests, and suggest a slight vertical vari-

ability across the formation in the range of 1 9 10-13 to

7 9 10-12 m s-1, possibly due to local variations of the

matrix structure, composition and cementation, or the

presence of fractures. In conclusion, the hydraulic con-

ductivity values of the deep borehole (DB) experiment

agree well with previous hydraulic testing results per-

formed in the hydraulically disturbed Opalinus Clay

around the Mont Terri rock laboratory. Therefore, future

hydraulic testing in the laboratory outside the excavated

damaged zone can be rated as comparable to undisturbed

conditions. However, results obtained in BDB-1 borehole

show higher values (in the order of 10-12 m s-1) for the

Opalinus Clay shaly facies than its sandy facies (in the

order of 10-13 m s-1), which is consistent with previous

microscopic observations (Peters et al. 2011). Further

laboratory experiments using Hassler cells will be per-

formed to characterise the Opalinus Clay permeability

anisotropy in the future.

Petrophysical analysis of drillcores and time-series

analyses are complementary to hydraulic testing. These

techniques involve different volumes of investigation. Core

analysis, as well as laboratory permeameter tests, give the

homogeneous matrix hydraulic properties but do not

account for larger scale heterogeneities such as sedimen-

tary and tectonic features. Moreover, analyses on core

Fig. 13 Compilation of results from hydraulic borehole packer tests

performed on the Opalinus Clay at the Mont Terri rock laboratory.

The high values of the shaly facies from previous studies might be

affected by the EDZ and are not quality assured (modified from

Lavanchy and Mettier 2012)

Mont Terri, paper #4: hydraulic parameters in Opalinus Clay 101

samples might be influenced by deconfining and alteration

of the core material, thus resulting in biased values.

Therefore, hydraulic testing in a fresh borehole is the

recommended method for determination of hydraulic con-

ductivity in overconsolidated clays. However, the pressure

perturbations e.g., induced by drilling activities have to be

taken into account for design and analyses of hydraulic

testing. The dissipation of drilling and installation of

instrumentation induced pressure pertubations can be

identified by the tidal components in the pore pressure time

series. Our study showed that drilling the BDB-1 borehole

with air as drilling fluid and a saturation with artificial

pore-water was an appropriate choice for our application,

because: (1) no mud-cake was created, (2) no artificial

osmotic effects and borehole convergence were observed

so far, (3) future water sampling can be carried out since

there was no contamination with drilling mud, and (4) we

reached fully undisturbed formation pressures after several

months. The latter was possible to do so in an underground

laboratory experiment, due no time and financial con-

straints, which are limiting factors on drill site for explo-

ration boreholes. Therefore, in clay formations, particular

care should be taken in the choice of drilling method and

drilling fluid as well as borehole instrumentation materials,

in order to obtain accurate hydraulic parameters.

Acknowledgements This study was performed in the framework of

the deep borehole (DB) experiment, financed by six partners of the

International Mont Terri Consortium (swisstopo, NAGRA, BGR,

GRS, NWMO, IRSN). The authors would like to thank Karam Kontar

and Jocelyn Gisiger (Solexperts AG) for their technical support and

realisation of hydraulic testing, as well as Christelle Courbet (IRSN)

and Benoıt Paris (INTERA) for advices on numerical interpretation.

The MuStat v1 package used in this paper is the result of previous

works respectively done by: Alain Mangin (CNRS, Laboratoire

d‘ecologie des hydrosystemes de Moulis), David Labat (Geosciences

Envionnement Toulouse), Rachid Ababou (CNRS/INPT/IMFT),

Hassane Fatmi (PhD at IRSN and CNRS/INPT/IMFT) and David

Bailly (TREES Institute). The constructive and careful reviews of

Prof. Z. Jiang (Queensland University of Technology, Brisbane,

Australia) and Prof. P. Cosenza (University of Poitiers, France)

contributed to improve the initial version of this article and are greatly

acknowledged.

Open Access This article is distributed under the terms of the

Creative Commons Attribution 4.0 International License (http://crea

tivecommons.org/licenses/by/4.0/), which permits unrestricted use,

distribution, and reproduction in any medium, provided you give

appropriate credit to the original author(s) and the source, provide a

link to the Creative Commons license, and indicate if changes were

made.

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